Let's break this down. We're going to add 6540 and 400 together.

Step 1: We'll start by adding the digits 0 & 0 in column 1 and get 0.

Step 2: We'll start by adding the digits 4 & 0 in column 2 and get 4.

Step 3: We'll start by adding the digits 5 & 4 in column 3 and get 9.

Step 4: We'll start by adding the digits 6 & 0 in column 4 and get 6.

So, 6540 + 400 = 6940.

